Japanese utility model publication No. 17010/1973 has proposed a method of measuring the flow speed or the flow rate of a fluid flowing through a conduit by disposing a vortex generating strut within the conduit and detecting the frequency of generation of Karman's vortices generated downstream thereof with an ultrasonic wave. With a Karman's vortex street detected by this method, an ultrasonic wave transmitted from an ultrasonic transmitter propagates through the fluid, is modulated with the Karman's vortex street and received by a receiver. However when the fluid has a low flow speed, the Karman's vortex street is weak and accordingly the degree of modulation undergone by propagated ultrasonic wave is small. Therefore it is difficult to detect the Karman's vortex street at low flow speeds. Further in the detection of the flow speed of fluids having changes in temperature occurring frequently therein resulting in an uneven temperature such as the air sucked into the carburetors of motor vehicles, the ultrasonic wave being propagated through the fluid is subjected, in addition to the modulation by the Karman's vortex street, to modulation due to a difference in sound velocity caused by the uneveness of the temperature. The modulation due to this uneveness of the temperature may make it difficult to detect the Karman's vortex street.